Errant readings on LM81 with T2080 SoC

[David Laight]

From: Linuxppc-dev Guenter Roeck
> Sent: 11 March 2021 21:35
> 
> On 3/11/21 1:17 PM, Chris Packham wrote:
> >
> > On 11/03/21 9:18 pm, Wolfram Sang wrote:
> >>> Bummer. What is really weird is that you see clock stretching under
> >>> CPU load. Normally clock stretching is triggered by the device, not
> >>> by the host.
> >> One example: Some hosts need an interrupt per byte to know if they
> >> should send ACK or NACK. If that interrupt is delayed, they stretch the
> >> clock.
> >>
> > It feels like something like that is happening. Looking at the T2080
> > Reference manual there is an interesting timing diagram (Figure 14-2 if
> > someone feels like looking it up). It shows SCL low between the ACK for
> > the address and the data byte. I think if we're delayed in sending the
> > next byte we could violate Ttimeout or Tlow:mext from the SMBUS spec.
> >
> 
> I think that really leaves you only two options that I can see:
> Rework the driver to handle critical actions (such as setting TXAK,
> and everything else that might result in clock stretching) in the
> interrupt handler, or rework the driver to handle everything in
> a high priority kernel thread.

I'm not sure a high priority kernel thread will help.
Without CONFIG_PREEMPT (which has its own set of nasties)
a RT process won't be scheduled until the processor it last
ran on does a reschedule.
I don't think a kernel thread will be any different from a
user process running under the RT scheduler.

I'm trying to remember the smbus spec (without remembering the I2C one).
While basically a clock+data bit-bang the slave is allowed to drive
the clock low to extend a cycle.
It may be allowed to do this at any point?
The master can generate the data at almost any rate (below the maximum)
but I don't think it can go down to zero.
But I do remember one of the specs having a timeout.

But I'd have thought the slave should answer the cycle correctly
regardless of any 'random' delays the master adds in.
Unless you are getting away with de-asserting chipselect?

The only implementation I've done is one an FPGA so doesn't have
worry about interrupt latencies.
It doesn't actually support clock stretching; it wasn't in the
code I started from and none of the slaves we need to connect to
ever does it.

	David

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